Electric motors are commonly used in various high power applications such as driving pumps, crane hoists and elevators. Such electric motors are driven by drive units, which change an input supply to a voltage and frequency required for driving the operation of the motor. Such drive units either have an AC or DC input for driving an AC motor. For DC-AC drive units the drive units have an inverter which converts the DC voltage to an AC voltage and frequency. For AC-AC drive units the drive units have a rectifier that converts the AC input voltage to a DC voltage, the DC voltage is then converted to an AC voltage using an inverter.
In both DC-AC drive units and AC-AC drive units the inverter can be controlled to vary the voltage and frequency characteristics of the output electricity supply used to drive the motor. It is common for each inverter to include a number of switch components, such as insulated gate bipolar transistors (IGBTs), for switching the DC voltage in such a way that produces an AC voltage for driving the motor.
In order to achieve high output powers for driving high power motors it is common to utilise multiple stacks connected in parallel. That is the drive unit may comprise multiple sets of parallel inverters each having a number of IGBTs. Such an arrangement is provided to accurately match the motor power requirements while reducing the power ratings of components within the drive unit.
In order to increase the life of individual inverters, there is a common desire to balance the current flow across each IGBT in a similar position in the topology between the individual stacks. It is common for current balancing methods to provide a correction every pulse width modulation (PWM) switching period. These methods typically act on each PWM switching period in turn and have no memory of any prior control action. These methods are commonly known as “per switching period” methods. Such methods can provide current balance at higher current flows but do not generally perform well at low current as the system gains and control action limits employed have to be low due to stability issues. These issues stem from the frequency at which the currents must be measured and hence are susceptible to noise and measurement errors.
One outcome of these “per switching period” methods is a large imbalance during periods of low current demand which results in one of the stacks supplying most of the power while the other stack(s) supply very little. In consequence, there is a difference in temperature and stress between the individual stacks. This often leads to premature faults in the hottest stack when the current demand increases, even though the majority of individual stacks are still below the fault, or trip, temperature.
Even when the current delivered to the output by each IGBT is well-balanced some IGBTs can degrade faster than others within the same drive unit due to differing thermal conditions, due to, for example, different cooling provisions. Even if all of the individual stacks are installed in the same cubical there may exist different temperatures, or hot spots, within the cubical. As such, even when the output currents are balanced there may still be differences between the stress imposed on other components within, or external to, the stacks, such as the inductive chokes.
Consequently, simply balancing the current flow through each IGBT is not necessarily a complete solution to increasing the life span and reliability of motor drive units.